A bioreactor is a vessel or container used for the growth of biological cultures, such as cells, bacteria, yeasts, or fungi. These cultures are used to produce a variety of biologically-active substances, including pharmaceuticals, fragrances, fuel, and the like.
A conventional bioreactor vessel has a cylindrical shape and includes a mixing apparatus, means for introducing a gas supply, and a temperature control system. Most bioreactor applications require a sterilization process to ensure that the process run does not yield the growth of, or contamination from, undesirable biological organisms. Traditional methods of sterilizing bioreactors include the use of an autoclave (for smaller vessels) or a steam sterilization-in-place process (for larger vessels). In recent years, there has been an introduction of single-use bioreactors. A conventional single-use bioreactor is typically delivered to the end user in the form of a pre-sterilized container and is used for only a single process run, after which the user disposes of the single-use bioreactor.
Typical materials of construction for small autoclavable bioreactor vessels may include, e.g., a glass vessel with a stainless steel cover, tube, and sensor adapters, mixing apparatus, and baffles. These vessels are filled with culture media and are then carried to an autoclave, where they are sterilized for several hours. The vessel is then taken back to the laboratory, where it is allowed to cool down. The vessel is then connected to a control system, which is used to control temperature, agitation, aeration and pH. The vessel is subsequently inoculated with the desired organism, at which time the process run proceeds.
While this has been the method of choice for process development for the last half century, the problem with this method is that the process is very time- and labor-consuming. Each process run typically lasts 1 to 10 days. Between each run, the vessel must be manually cleaned and sterilized in the autoclave. This can result in a turnaround time of 1 to 3 days between runs. As a result, the single-use bioreactor was developed to allow the end user to eliminate the sterilization and cleaning portion of the process. At the end of each run, the vessel is discarded, and a new pre-sterilized vessel is removed from the packaging and is set up to start the next run.
A typical single-use bioreactor employs a medical-type bag that rests on top of a platform, where an orbital or rocking motion is induced to mix and aerate the organism. However, a problem with this type of system is that the mixing action and oxygen transfer are limited, resulting in a low organism growth density relative to the design of a traditional agitator and sparger (a device that bubbles gas through the bioreactor) in a “stirred tank” bioreactor. In addition, the different mixing motion and bag geometry in the medical-type bag bioreactor does not lend itself to be scaled up to larger production processes. Further, in order for a scientist to use one of the medical-type bag single-use rocking systems, the scientist is required to invest substantial capital and laboratory space in an additional piece of specialized equipment.
These limitations of the initial single-use bioreactor designs have led to the development of “stirred-tank” single-use bioreactor products.
One type of stirred-tank single-use bioreactor product employs a bag-type vessel containing a mixer, sparger, sensors, and tubing. The bag is placed into a structure of some type (typically a stainless cylinder) and conforms to its shape. The intent is for the bag to mimic a traditional stainless-steel or glass stirred-tank bioreactor. However, the design does not lend itself to small vessel sizes (e.g., less than 50 L), because the size of the fittings in the bag and the number of fittings for a standard bioreactor exceed the allowable accessible surface area on the bag.
Another type of stirred-tank single-use bioreactor product employs a rigid plastic vessel that is self-supporting and has a self-contained mixing apparatus, aeration system, and sensors. This type of unit mimics the look and feel of a traditional stainless-steel or glass-type bioreactor vessel. However, unlike a single-use bag system, the rigid plastic used in this design cannot be folded or reduced in size for storage or disposal. This burdens the end user with a requirement for a large environmentally-controlled area within the user's facility for storage of future vessels, as well as increased disposal costs associated with such larger, non-compressible vessels.